The management of the flow of liquids within small diameter channels presents challenges as the scale of the channels and volumes of the liquids are reduced. One significant constraint is the configuration of traditional valve technology. The need to manage the flow of liquids in nano-liter volumes encounters significant limitation when the scale of fluid management is severely affected by dead space volume that is inherent within traditional switching methods. The method of using fluid within these nano scale capillaries and channels to act as an on/off valve by freezing and thawing that liquid is known in the art, see for example U.S. Pat. Nos. 6,159,744 and 5,795,788. It has been found that the flow of liquids can be stopped or diverted to a further channel or chamber by merely freezing and thawing the liquid contained within a segment of tubing or channel. This flow-switching device that is referred to as “freeze thaw valving,” requires no moving parts and most importantly contributes no dead volume within the analytical system.
Prior art freeze thaw valves freeze the liquid within a freeze thaw segment by providing a jet of cold gas from a liquefied source of gas under pressure directly onto the freeze thaw segment. Pressurized gases such as liquid carbon dioxide and liquid nitrogen have been used to freeze the contents of the freeze thaw segment. Unfortunately, these cryogenic agents are aspirated directly at the freeze thaw segment without any method of recovery or limitation. The flow of these cryogenic agents without any limitation greatly increases the volume of their use and consequently the expense associated therewith. Further, the volume of cryogen necessary in prior art methods not only adds to the expense of their use within analytical systems, but also, because of the needed volumes the use of freeze thaw valving in small bench top or portable instruments is impractical.
Additionally, the prior art method of directing these agents at the freeze thaw segment, which is exposed to ambient air, causes the accumulation of frost buildup. This frost buildup forms an insulting layer that reduces exposure of the freeze thaw segment to the cryogen causing an increase in temperature within interior channels of the freeze thaw segment resulting ultimately in the failure of the freeze thaw valve. In addition, this frost accumulation on the freeze thaw valve during its closed operation is converted into liquid water upon heating/opening of the valve. If a freeze-thaw valve is used in an instrument, methods of draining this water away from sensitive electrical components in the instrument must be used.